Inverters have been employed in many types of electrical equipments, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, followed by applying a voltage twice as high as the inverter output voltage at the maximum. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and voltage decay due to the connection cable is also low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipments, such as inverter-related equipments, for example, high-speed switching devices, inverter motors, and transformers, insulated wires made of enameled wires are mainly used as magnet wires in the coils. Further, as described above, since a voltage almost twice as high as the inverter output voltage is applied in inverter-related equipments, it is required in insulated wires to have minimized partial discharge deterioration, which is attributable to inverter surge.
In general, partial discharge deterioration is a phenomenon in which an electrical insulating material undergoes, in a complicated manner, for example, molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge of the electrical insulating material (discharge at a portion in which fine void defect exists), sputtering deterioration, thermal fusion or thermal decomposition deterioration caused by local temperature rise, and chemical deterioration caused by ozone generated due to discharge. For this reason, reduction in thickness, for example, is observed in the actual electrical-insulation materials, which have been deteriorated as a result of partial discharge.
In order to prevent deterioration of an insulated wire caused by such partial discharge, insulated wires having improved resistance to corona discharge that is acquired by incorporating particles into an insulating film have been proposed. For example, an insulated wire containing metal oxide microparticles or silicon oxide microparticles incorporated into the insulating film (see Patent Literature 1), and an insulated wire containing silica incorporated into the insulating film (see Patent Literature 2) have been proposed. These insulated wires reduce erosive deterioration caused by corona discharge, by means of the insulating films containing particles. However, these insulated wires having an insulating film containing particles have a problem that a partial discharge inception voltage is decreased, or flexibility of a coating film is decreased.
There is also available a method of obtaining an insulated wire which does not cause partial discharge, that is, an insulated wire having a high partial voltage at which partial discharge occurs. In this regard, a method of making the thickness of the insulating layer of an insulated wire thicker, or using a resin having a low relative dielectric constant in the insulating layer can be considered.
However, when the thickness of the insulating layer is increased, the resultant insulated wire becomes thicker, and as a result, size enlargement of electrical equipments is brought about. This is retrograde to the demand in recent miniaturization of electrical equipments represented by motors and transformers. For example, specifically, it is no exaggeration to say that the performance of a rotator, such as a motor, is determined by how many wires are held in a stator slot. As a result, the ratio (space factor) of the sectional area of conductors to the sectional area of the stator slot, has been required to be particularly highly increased in recent years. Therefore, increasing the thickness of the insulating layer leads to a decrease in the space factor, and this is not desirable when the required performance is taken into consideration.
On the other hand, with respect to the relative dielectric constant of an insulating layer, most of resins that are generally used as a material for the insulating layer have a relative dielectric constant from 3 to 4, and thus there is no resin having a specifically low relative dielectric constant. Furthermore, in practice, a resin having a low relative dielectric constant cannot always be selected necessarily when other properties that are required for the insulating layer (heat resistance, solvent resistance, flexibility and the like) are taken into consideration.
As a means for decreasing the substantial relative dielectric constant of the insulating layer, such a measure is studied as forming the insulating layer from foam, and foamed wires containing a conductor and a foamed insulating layer have been widely used as communication wires. Conventionally, foamed wires obtained by, for example, foaming an olefin-based resin such as polyethylene or a fluorine resin have been well-known. As examples of such foamed wires, foamed polyethylene insulated wires are described in Patent Literature 3, foamed fluorine resin insulated wires are described in Patent Literature 4, the both insulated wires are described in Patent Literature 5.
However, since such a foamed wire has a low heat resistant temperature of the coating film and poor scratch resistance, it is not satisfactory from this point of view.